Obstinate article penned by unusually tart cleric (12)
I believe the answer is:
recalcitrant
'obstinate' is the definition.
(I've seen this before)
'article penned by unusually tart cleric' is the wordplay.
'article' becomes 'an' (indefinite article).
'penned by' means one lot of letters goes inside another.
'unusually' is an anagram indicator.
'tart'+'cleric'='tartcleric'
'tartcleric' is an anagram of 'recalcitrt'.
'an' placed within 'recalcitrt' is 'RECALCITRANT'.
(Other definitions for recalcitrant that I've seen before include "Refractory" , "Obstinately disobedient - cartel in cart (anag)" , "Wilfully disobedient" , "Obstinately uncooperative" , "Defiant" .)